Quantum key distribution is a technique that generates random and secure private digital data between two parties or among three or more parties. The digital data is called key data. Key data can be used for encryption in communication performed between the parties. The parties normally mean transmitters or receivers.
According to quantum key distribution, key data is transmitted from a transmitter to a receiver as quantum information. Thus, the maximum amount of information that may be acquired by a third party eavesdropper as key data can be estimated based on quantum theory. However, this estimation assumes that the transmission source of key data is identified.
Some errors arise in transmission of quantum information. That is, key data received by a receiver does not perfectly match key data transmitted by a transmitter. Thus, it becomes necessary to correct any errors contained in key data before the key data is used between parties. An error may be caused by noise in a transmission channel or by the interception of key data by an eavesdropper.
Errors contained in key data are typically corrected by using a technique called the forward error correction. According to the forward error correction, a receiver corrects errors contained in reception key data so that the reception key data matches transmission key data. Therefore, when the forward error correction is applied, final secure key data matches transmission key data.
On the other hand, a technique called the backward error correction is also known. In the backward error correction, a difference of transmission key data from reception key data is handled as an error. That is, according to the backward error correction, a transmitter corrects errors contained in transmission key data so that the transmission key data matches reception key data. Therefore, when the backward error correction is applied, final secure key data matches reception key data.
In quantum key distribution, processing called privacy amplification is provided after error correction. Information that may be acquired by an eavesdropper of key data shared among normal parties is removed by privacy amplification. More specifically, normal parties discard some amount of key data and also remove information that may be acquired by an eavesdropper by reducing the size of the key data using a hash algorithm.